A retrospective study evaluating posttreatment symmetry in dental arch form and midlines was carried out in Class II subdivision patients treated with unilateral and bilateral maxillary premolar extractions.
Using Geomagic (version 14; Geomagic, Research Triangle Park, NC) and MATLAB (version 8.4; MathWorks, Natick, Mass) software, best-fit curves expressed as quartic polynomials were generated for 13 Class II subdivisions treated with unilateral extractions and 20 treated with bilateral maxillary premolar extractions. Transverse and sagittal measurements were recorded to assess symmetry. Dental models were superimposed on constructed reference planes to generate average posttreatment arches. Statistical comparisons were performed with the significance level set at P ≤0.05.
The unilateral extraction group showed significant differences in transverse arch forms between the right and left sides in the anterior, anterior-middle, and middle segments of the arch, and all regions other than the posterior segment in the sagittal dimension. Significant differences were found between groups in the anterior and anterior-middle segments of the arch transversely, the middle and middle-posterior segments sagittally, and the midline deviation relative to the midsagittal plane. Superimposed average arches showed similar results.
Unilateral maxillary extraction treatment generally results in a narrower and more posteriorly displaced arch form on the extraction side, with a deviated maxillary midline toward the extraction side of the arch.
Maxillary arches were evaluated at posttreatment in Class II subdivision malocclusions.
Unilateral extractions showed asymmetric transverse and sagittal differences in arch form.
Unilateral extraction results in a narrow posteriorly displaced arch on the extraction side.
In unilateral extraction patients, the maxillary midline deviates toward the extraction side.
Treatment of Class II subdivision malocclusions is challenging and requires a thorough analysis of the source of the asymmetry—whether it is in the maxilla, the mandible, or both. Further investigation must be done to identify whether a skeletal component is involved. The entire maxilla may be positioned farther forward on the Class II side, or the mandible could be retruded, or there could be a combination of the two. Also, a combination of disharmony in skeletal relationships and the denture to skeletal relationships may exist in the same patient.
Evidence shows that the primary element contributing to a Class II subdivision is the distal positioning of the mandibular first molar on the Class II side, categorized as a type 1 malocclusion. The type II category is characterized by the mesial positioning of the maxillary first molar, which is a secondary contributor. Findings from a study using cone-beam computed tomography showed that the primary cause may be skeletal: ie, an asymmetric mandible that is shorter and more posteriorly positioned on the Class II side.
Asymmetric orthodontic mechanics can successfully correct minor asymmetries. Movements are limited, and high levels of unwanted side effects can result if mechanics are exploited unreasonably. Correction with functional appliances seems sensible, yet there are insufficient studies on the treatment outcomes using these approaches in asymmetric malocclusions. Dental asymmetries resulting from altered molar axial inclinations can be corrected using unilateral tip-back moments. Unilateral application of intermaxillary elastics has been linked to skewed arch forms, asymmetric overjet, mandibular incisor flaring, and canting of the occlusal plane.
A unilateral extraction may be a viable treatment option in malocclusions of moderate to severe asymmetry in which surgery is contraindicated or rejected by the patient. By creating space asymmetrically, the practitioner can conduct the mechanics of treatment in a symmetric fashion and avoid many of the side effects encountered with asymmetric mechanics. In-depth analysis of anchorage requirements should be conducted beforehand so that the teeth extracted allow for maximum chances of proceeding with treatment in a symmetric fashion. Other benefits include maintenance of existing molar relationships, reduced treatment time, and greater ease of midline correction without canting of the occlusal plane.
The complex nature of these malocclusions may be why there are so few studies related to the treatment and outcomes of these patients in the literature. The purpose of this study was to contribute details on the outcomes of treatment strategies implemented for these patients: specifically, unilateral and bilateral maxillary premolar extractions. The posttreatment effects on maxillary dental arch form and midline deviation relative to the midpalatal raphe were both evaluated. Since extraction treatment has been shown to alter arch width, it is within reason to speculate whether extracting 1 premolar unilaterally would produce a different response in the arch form between the extraction and nonextraction quadrants.
Material and methods
This study was approved by the Institutional Review Board of the University of Illinois at Chicago. Deidentified records (intraoral photographs, digital models, and standard forms summarizing diagnosis and treatment) of Class II subdivisions treated with unilateral and bilateral premolar extractions were examined by the principal investigator (G.D.). The criterion to classify as a Class II subdivision malocclusion was an Angle Class I molar relationship on 1 side of the dentition with at least a half step Angle Class II molar relationship on the contralateral side. Patients with lateral mandibular functional or habitual shifts were excluded from the study. Subjects with significant contralateral tooth-size discrepancies (ie, unilateral peg lateral), many malformed or missing teeth (excluding third molars), or teeth with extensive restorations or gross decay were excluded. Those with cleft lip and palate, facial trauma, severe facial asymmetries, or syndromes associated with craniofacial deformities were excluded. Any patients debonded before achieving optimal occlusion were also removed from the sample. Thirteen subjects met the inclusion criteria for the experimental group (unilateral premolar extraction) and 20 for the control group (bilateral premolar extractions).
Analysis was limited solely to the effects on the maxillary arch. Among the experimental subjects, 9 patients were treated with 1 maxillary first premolar extraction, and 4 with 1 maxillary second premolar extraction. One patient in this group also had 1 mandibular premolar extracted along with the maxillary premolar extraction on the same side. Seven of the 13 subjects had 1 premolar extracted on the right side of the maxillary arch, and 6 subjects had 1 premolar extracted on the left. In the control group, the same tooth was extracted on the right side of the arch as was extracted on the left side; treatment of 16 of these subjects included bilateral mandibular premolar extractions.
Stereolithography files of posttreatment digital models were imported into Geomagic Control (version 14; Geomagic, Research Triangle Park, NC) for construction of reference planes and landmark digitization. The horizontal (x-y) plane was constructed by digitizing the facial axis (FA) points from first molar to first molar and constructing a best-fitting plane that ran through these points. The FA point was defined as the center of the facial axis of the clinical crown. For the first molar, the FA point was defined as the point midway occlusogingival along the mesiobuccal groove. To construct the midsagittal plane (y-z), 2 points were selected to create a line (y-axis) running along the anteroposterior direction. The midpoint between the medial ends of the first palatine rugae was selected as the anterior point, and the most posterior point visible on the midpalatal raphe was chosen as the posterior point. The yz plane was constructed running through this line, perpendicular to the x-y plane. For the construction of the coronal (x-z) plane, a point was selected in the middle of the incisive papilla (anteroposteriorly) that fell on the y-axis line. The plane was constructed perpendicular to the other 2 planes running through this point. The coordinate grid system in Geomagic was aligned with the 3 constructed planes of reference with the origin at the intersection of these planes ( Fig 1 ).
The point coordinate tool in Geomagic was used to digitize the FA points for each tooth on the virtual models from the right to the left first molars. The 3-dimensional coordinates of these landmarks were imported into MATLAB (version 8.4; MathWorks, Natick, Mass), in which a coded program was designed to use these coordinates and construct the best-fitting curve that represents the dental arch form. The dental arch form was expressed as a quartic polynomial expression. An example of a best-fit curve generated by the program can be seen in Figure 2 . The software program was designed to make measurements in the transverse and sagittal dimensions for assessment of symmetry of the constructed dental arch form.
For analysis in the transverse dimension, the distance from the minimum to the maximum y value was divided into 15 equal segments. Data collection began at the subsequent y value after the minimum y value, for a total of 15 preselected y values. The values of x on either side of these preselected y values were interpolated. The x value was extrapolated for the last point on the side of the arch that ran shorter. The right and left values of x at each preselected y value and the difference between these were provided by MATLAB.
For analysis in the sagittal dimension, the MATLAB program identified the narrower side of the arch at the maximum y value and divided the distance into 10 equal segments. The segment length was calculated, and this distance was used to create segments of equal lengths in the opposite or wider side of the arch. MATLAB performed a comparison between the numbers of segments on each side of the arch and eliminated the extra segment on the wider side to give an equal number of segments of equal sizes on both sides of the arch. Data collection began in the right and left directions after x = 0 for a total of 10 preselected x values on each side of the arch. The values of y on both sides of the arch were interpolated at these preselected x values. These y values and the difference between them were given by the software.
For the purposes of describing the arch form in this study, each maxillary dental arch was divided into 5 segments in both the transverse and sagittal dimensions ( Fig 3 ). The segments were labeled as follows: anterior, anterior-middle, middle, middle-posterior, and posterior. The 15 measurements in the transverse dimension were divided into groups of 3, and these measurements were averaged to give 5 values that were related to their corresponding segment region on the arch. Similarly, the 10 measurements in the sagittal dimension in each half of the arch were divided into groups of 2, and these measurements were averaged to give 5 values that were related to their corresponding areas on the arch.
To study the deviation of the dental midline relative to the midpalatal raphe in the transverse dimension, the absolute x values of the FA-points of the maxillary central incisors were recorded for each subject. In the unilateral extraction group, FAx1 and FAx2 denoted the absolute x values of the digitized FA points of the maxillary central incisors on the nonextraction and extraction halves of the arch, respectively. The averages of these values were computed, and the distance of this average value from the y-axis was calculated to determine the deviation of the maxillary midline. Arches from each group were superimposed in Geomagic software on the constructed axes and planes to generate 1 arch that was a qualitative representation of the average posttreatment dental arch form of all arches in each group. Arches with unilateral extractions on the left side were flipped on the y-z plane to create a mirror model. This ensured uniformity of the extraction region before the superimpositions. Gingival and palatal tissues were manually selected and eliminated for each arch to prevent introduction of extra noise and distortion during data processing. All fixed lingual retainers were also manually selected for elimination for similar reasons, and the void was filled using a new mesh that matched the curvature of the surrounding mesh. Third molars were not included in the superimpositions. The constructed average arch from each group was quantitatively analyzed in Geomagic and MATLAB software using methods similar to those described earlier.
To evaluate intraoperator reliability of construction of reference planes and dental landmark identification, 5 digital models were randomly selected and reassessed 2 weeks later by the same operator. The planes were reconstructed, and the FA points were redigitized to obtain the x, y, and z coordinates. Interoperator reliability was examined between 2 operators (G.D., A.M.). The intraclass correlation coefficient was used to test the intraoperator and interoperator reliability values.
The distribution of the raw data was investigated with the Shapiro-Wilk test of normality. Student t tests were used to test the mean difference in each experimental group and the mean differences between the 2 experimental groups for all variables in the study. Statistical significance was set at 0.05. Data analysis was performed using SPSS Statistics for Windows (version 22.0; IBM, Armonk, NY).
Intraclass and interclass correlation coefficients were higher than 0.95 for all variables in the study and showed good support for the reliability of the method we used. All variables involving arch-form measurements were normally distributed according to the Shapiro-Wilk test of normality. Measurements involving midline deviation were normally distributed only for the test group according to the Shapiro-Wilk test of normality. The data for the control group did not show a normal distribution. Nonparametric tests were performed (Mann-Whitney), and similar conclusions to those of parametric testing were drawn. As a result, all findings are reported from parametric tests.
Paired t tests were performed to test the mean difference between corresponding values on the right and left sides of the arch in each experimental group. Independent Student t tests were used to test the mean difference between the test and control groups of the variables in the study.
The unilateral extraction group showed statistically significant mean differences between the extraction and nonextraction quadrants in the anterior, anterior-middle, and middle segments of the arch in the transverse dimension, and all segments of the arch other than the posterior segment in the sagittal dimension ( P <0.05). Table I summarizes the test results. For patients treated with bilateral premolar extractions, no significant differences were found statistically in the width of analogous points between the right and left quadrants of the dental arch in the transverse and sagittal dimensions. Table II summarizes the test results. Between the 2 groups, the anterior and anterior-middle segments showed statistically significant mean differences in the transverse dimension, and the middle and middle-posterior segments in the sagittal dimension ( P <0.05). Table III summarizes the test results.
|Variable||n||Mean||SD||SE mean||95% CI of the difference||Significance (2-tailed) ∗|
|Variable||n||Mean||SD||SE mean||95% CI of the difference||Significance (2-tailed) ∗|